Vaso-occlusive devices or implants are used for a wide variety of reasons. They are often used for treatment of intra-vascular aneurysms. This is to say that the treatment involves the placement of a vaso-occlusive device in an aneurysm to cause the formation of a clot and eventually of a collagenous mass containing the vaso-occlusive device. These occlusions seal and fill the aneurysm thereby preventing the weakened wall of the aneurysm from being exposed to the pulsing blood pressure of the open vascular lumen. Treatment of aneurysms in this fashion is a significant improvement over the surgical method typically involved.
A common vaso-occlusive device is a soft, helically wound coil. A typical commercial coil will be formed by winding a platinum wire strand about a primary mandrel and applying a heat treatment to impart a primary winding coil shape. The relative stiffness of the coil will depend, among other things, on the diameter of the wire strand, the diameter of the primary mandrel, and the pitch of the primary windings. The device is then wrapped around a secondary mandrel, and again heat treated to impart a secondary shape. For example, U.S. Pat. No. 4,994,069, to Ritchart et al., describes a vaso-occlusive coil that assumes a primary, linear helical configuration when stretched and a folded, and a convoluted, secondary configuration when relaxed in a minimal energy configuration. The stretched condition is used in placing the coil at the desired site (by its passage through a delivery catheter) and the coil assumes a relaxed configuration—which is better suited to occlude the vessel—once the device is so placed.
It is well-known to detach such vaso-occlusive coil devices from a delivery wire using a mechanical detachment mechanism. For example, U.S. Pat. No. 5,234,437, to Sepetka, shows a method of unscrewing a helically wound coil from a pusher having interlocking surfaces. U.S. Pat. No. 5,250,071, to Palermo, shows an embolic coil assembly using interlocking clasps mounted both on the pusher and on the embolic coil. U.S. Pat. No. 5,261,916, to Engelson, shows a detachable pusher-vaso-occlusive coil assembly having an interlocking ball and keyway-type coupling. U.S. Pat. No. 5,304,195, to Twyford et al., shows a pusher-vaso-occlusive coil assembly having an affixed, proximally extending wire carrying a ball on its proximal end and a pusher having a similar end. The two ends are interlocked and disengage when expelled from the distal tip of the catheter.
It is also well-known to use an electrolytically severable joint to release vaso-occlusive coils at the vessel site. For example, Guglielmi et al. shows an embolism forming device and procedure for using that device which employs an electrolytically severable joint. Specifically, Guglielmi et al. desirably places a finely wound platinum coil into a vascular cavity such as an aneurysm. The coil is delivered endovascularly using a catheter such as those described above. After placement in the aneurysm, the coil is severed from its insertion core wire by the application of a small electric current to that core wire. The deliverable coils are said to be made of a platinum material. Proximal of the embolic coil, as noted above, is a core wire which is typically stainless steel. The core wire is used to push the platinum embolic coil into vascular site to be occluded. Other variations of the Guglielmi et al. technology are found in U.S. Pat. No. 5,354,295.
Current electrolytically detachable coil products employ a relatively inflexible bridge assembly that connects the proximal end of the vaso-occlusive coil to the distal end of the pusher wire assembly. When the coil is detached from the pusher wire, the force the pusher wire has been exerting on the coil (and aneurysm wall) pushes back on the pusher wire assembly, which can displace the tip of the introducer catheter out of the aneurysm. This is because the PET sleeve does not laterally buckle or flex, but instead axially transmits the push-back force against the distal tip of the delivery catheter. Having the catheter tip displaced from the aneurysm requires the physician to relocate the catheter tip prior to placement of a further occlusive device, which undesirably extends the duration of the procedure.